Without limiting the scope of the invention, the background will describe surface controlled, subsurface safety valves, as an example.
Surface controlled, subsurface safety valves are commonly used to shut in oil and gas wells in the event of a failure or hazardous condition at the well surface. Such safety valves are typically fitted into the production tubing and operate to block the flow of formation fluid upwardly therethrough. The subsurface safety valve provides automatic shutoff of production flow in response to a variety of out of range safety conditions that can be sensed or indicated at the surface. For example, the safety conditions include a fire on the platform, a high or low flow line temperature or pressure condition or operator override.
During production, the subsurface safety valve is typically held open by the application of hydraulic fluid pressure conducted to the subsurface safety valve through an auxiliary control conduit which extends along the tubing string within the annulus between the tubing and the well casing. Flapper type subsurface safety valves utilize a closure plate which is actuated by longitudinal movement of a hydraulically actuated, tubular piston. The flapper valve closure plate is maintained in the valve open position by an operator tube which is extended by the application of hydraulic pressure onto the piston. A pump at the surface pressurizes a reservoir which delivers regulated hydraulic control pressure through the control conduit. Hydraulic fluid is pumped into a variable volume pressure chamber and acts against the crown of the piston. When, for example, the production fluid pressure rises above or falls below a preset level, the control pressure is relieved such that the piston and operator tube are retracted to the valve closed position by a return spring. The flapper plate is then rotated to the valve closed position by a torsion spring or tension member.
It has been found, however, that in tight safety valve applications having a large inner diameter and a small outer diameter, typical torsion spring or tension member designs provide insufficient bias or closure force to lift the flapper plate to the closed position. In the case of a torsion spring, the size and therefore the closure force of the torsion spring are limited by the space available to house the torsional spring. In the case of a tension member, closure force is limited by length of the lever arm between the hinge pin of the flapper plate and the location where in the tension member is attached to the linkage that extends from the hinge in the direction opposite of the flapper plate.
Therefore, a need has arisen for a flapper valve that has sufficient bias or closure force to lift the flapper plate into the closed position in tight applications. A need has also arisen for such a flapper valve the can produce the required closure force without increasing the space available for a spring within the flapper valve. A need has further arisen for such a flapper valve that can take advantage of a longer lever arm to exert a greater closure force to the flapper plate.